Abstract: A method which uses the measured modes and natural frequencies to/dentify structural parameters is formulated by selecting a measure of difference between analytical pa-rameters and those to be identified and using Lagrange Multipliers for minimizing changes in structural parameters to meet the constraint condition of modal orthogonality and the eigenval-ue equation.Examples are presented to illustrate how this method is applied to the models in which there are freedoms that can not be measured or that are reduced.The results of examples show that relatively large deviations between analytical natural frequencies and measused ones can be improved with this method,and that this method is effective and has a good accuracy.